Thymic selection involves conversion of a continuous antigen receptor signal to a discrete set of developmental outcomes: death, positive selection or negative selection. This highly regulated developmental process plays a critical role in the selection of a population of T cells carrying a variety of functional T cell receptors (positive selection), while eliminating autoreactive T cells (negative selection). The class II histone deacetylase HDAC7 is expressed at high levels in the nucleus of double positive thymocytes. HDAC7 is exported from the nucleus to the cytoplasm in response to T cell receptor activation and allows changes in gene expression associated with positive selection. Loss of stageappropriate gene expression in HDAC7-deficient DP thymocytes shortens their survival in vivo and impairs maturation to the single positive stage. Transgenic expression of an HDAC7 mutant that cannot be exported from the nucleus increases single positive thymocyte numbers and renders thymocytes resistant to negative selection. Mice expressing this HDAC7 mutant protein in the thymus develop a striking autoimmune exocrine pancreatitis. Finally, both knockout and transgenic mice show distinct and abnormal distributions of CD4 effector T cells. These observations support the model that HDAC7 participates in the transcriptional state changes associated with both positive and negative selection and the subsequent differentiation of CD4 na[unreadable]ve T cells in effector T cells. We propose: 1. To map the transcriptional cascade downstream of HDAC7 in thymocytes. We will use chIP-seq to identify the direct primary targets of binding of HDAC7 in the genome. This gene list will be compared with the genes that are transcriptionally activated when HDAC7 is knocked out in vivo. This analysis will be followed by pathway analysis both in silico and in the laboratory to establish the connectivity of the primary and secondary targets of HDAC7. 2. To characterize the autoimmune phenotype of transgenic mice expressing a dominant positive HDAC7 mutant protein (HDAC7-&#56256;&#56321;P). We have observed a highly penetrant autoimmune exocrine pancreatitis syndrome in transgenic mice expressing a mutant HDAC7 protein (HADC7-&#56256;&#56321;P) that is not exported from the nucleus in response to TCR signaling. HADC7-&#56256;&#56321;P may allow autoreactive T cells to persist by suppressing the death of cells that normally undergo negative selection. However, a disturbance in the balance of effector and regulatory T cell types in the periphery is another possible mechanism. We propose to further characterize this phenotype and in particular to define the contribution of HDAC7 to the induction of central vs. peripheral tolerance. 3. To examine the physiological function of HDAC7 in murine models of T cell differentiation. After exiting the thymus, Na[unreadable]ve CD4+ T cells can undergo further differentiation in the periphery into distinct effector T cell subsets, depending on TCR activation as well as signaling through specific cytokine receptors. Perturbing the balance among these effector CD4+ T cells can lead to inflammation and/or autoimmunity, by inappropriate expansion of pro-inflammatory T cells and by insufficient production and expansion of regulatory T cells. We have obtained several lines of evidence supporting the hypothesis that HDAC7 also plays an important role in the regulation of effector T cell differentiation in the periphery in addition to its specific effects in the thymus. We propose to functionally characterize the role of HDAC7 in effector T cell differentiation in mice, to determine the effect of perturbation of HDAC7 function on changes in gene expression during effector T cell differentiation and to examine the role of HDAC7- mediated effector differentiation in murine disease models.